There is a need to increase the current switched through many semiconductor power supplies. A common practice in power electronics is to use a parallel arrangement of standard size power switching devices, such as bipolar transistors, field effect transistors (FETs), insulated field-effect transistors (IGFETs), and insulated gate bipolar transistors (IGBTs) instead of a single large power device, in order to handle the high current and power dissipation that may otherwise exceed the safety ratings of a single power switching device. Also, it is a common practice in power electronics to use a parallel arrangement of diode rectifiers to handle high currents that would otherwise exceed the safety ratings of a single diode rectifier.
The use of a parallel arrangement of standard size power devices is preferred over the alternative of using a single larger power device for the following reasons: (1) a parallel arrangement of standard size power devices is usually less expensive than a single high power device because standard size power devices are built in high quantities and sold at competitive prices whereas high power devices are made in smaller quantities and sold at less competitive prices; (2) smaller components tend to switch faster and more efficiently; (3) the packaging and thermal arrangement of standard size power devices is easier than for high power devices; and (4) an appropriate single high power device is sometimes not available.
Unfortunately, the use of parallel power devices has its own set of disadvantages. First, there is no guarantee of equal current sharing between the parallel power devices. To achieve the best performance and lowest temperature rise it is necessary that current be shared roughly equally between power devices coupled in parallel. As a result, larger power rated devices need to be used in order to provide a sufficient operating safety margin. Second, when two or more parallel power devices are switched, the power device that turns on the fastest will absorb all of the turn-on losses. Similarly, the power device that turns off the slowest will absorb all of the turn-off losses. Therefore, the switching loss, both turn-on and turn-off, is not shared equally among the parallel power devices and, consequently, there is a higher power dissipation in individual power devices. Third, in the case of power rectifier devices, such as silicon and Schottky diodes, which have a forward voltage with a negative temperature coefficient, it is fairly easy for one of the paralleled rectifiers to go into thermal runaway.
FIG. 1 shows a schematic diagram of a prior art boost power converter 100 using FETs as parallel power switching devices and parallel diode rectifiers to provide a circuit that will handle high currents that would exceed the safety ratings of a boost converter that used only a single switch and a single diode rectifier. Voltage V.sub.in is input into boost converter 100 at input terminals 102. The input voltage V.sub.in is then boosted by inductor 104 and an output voltage V.sub.out is output at output terminals 103 by way of output capacitor 114. Boost converter 100 preferably uses two FET power switches 106 and 108. A pulsed signal source 101 is used to drive the switches 106, 108. In switches 106, 108, the load current in each switch will depend on the channel resistance of the FET. Consequently, for similar FETs, current will be shared reasonably well when these devices are connected in parallel. As shown in FIG. 1, additional protective resistors, R, are preferably used to couple the gates of switches 106, 108 to signal source 101 in accord with common circuit practice. Two diode rectifiers 110 and 112 connected in parallel are also used.
A key limitation to using parallel diodes 110, 112, and to a lesser extent FET switches 106 and 108, is that equal current sharing typically does not occur. One of the two devices will tend to draw substantially more current than the other. Consequently, the circuit designer must design the parallel switches or diodes assuming a large variation in nominal current, i.e., the circuit designer must assume that each of the two power device draws 50%.+-.X%, of the total current where X accounts for the fact that unequal current sharing occurs. For example, one device may draw 66% of the current and the other 34% of the current. If X is large, the circuit designer will be forced to use larger current value diodes or transistors than if substantially equal current sharing were enabled. Alternatively, the designer may be forced to limit the maximum current to less than what would be possible if substantially equal current sharing occurred.
One reason why equal current sharing does not occur between parallel diodes 110, 112 or parallel switches 106, 108 is that there are inherent manufacturing tolerances associated with each device. For example, typically the turn-on voltage and forward resistance of two diode rectifiers varies by at least several percent. Consequently, even though the same voltage is applied across parallel diodes 110, 112, different currents will flow through each diode 110, 112. Similarly, there are slight manufacturing variations in the source-to-drain channel resistance of FET switches 106, 108.
In many cases, thermal affects exacerbate inherent manufacturing variations. Semiconductor devices have current-voltage characteristics that are temperature dependent. For example, if there are slight variances in the current-voltage characteristics of parallel diodes 110, 112 one of the diodes will draw slightly more current than the other. The diode that draws the largest current, i.e., the one with the lowest turn-on voltage and on-resistance, will tend to operate at a higher temperature. However, the turn-on voltage and forward resistance of a diode rectifier decreases with temperature. Device heating will thus tend to exacerbate initial manufacturing variances in a diode's current-voltage characteristics, with the unwanted result of thermal runaway as most of the current gets diverted through the dominant diode.
A further reason why equal current sharing does not occur is that parallel devices may not switch perfectly in phase. There is a manufacturing tolerance associated with the inductance and capacitance of each switching device. Moreover, there are manufacturing tolerances associated with the packaging elements used to interconnect individual switching devices to a signal source. Consequently, while switches 106, 108 may be driven substantially in phase by the same signal source, there may be slight differences in their actual turn-on and turn-off responses. Consequently one device may absorb a disproportionate share of the turn-on or turn-off switching losses.
The problems associated with the switches 106, 108 of FIG. 1 are exacerbated when bipolar transistors are used instead of FETs for these switches. For example, as shown in FIG. 2, two bipolar switches 116, 118 may be used as parallel switches. However, bipolar transistors have nonlinear voltage/current characteristics and a negative temperature coefficient of "on" voltage. It is therefore common practice to insert an additional resistor 120, 122 in series with respective emitter terminals of transistors 116, 118 to reduce the effect of variations in the current-voltage characteristics of each transistor in order to prevent one transistor from drawing substantially more current than the other one. This increases component count, circuit size, and cost. Similarly, as shown in FIG. 3, two parallel rectifiers 124, 126 may have resistors 128, 130 in series with each individual diode. Still another prior art approach, as shown in FIG. 4, is to drive parallel diodes 132, 134 from secondary windings 142, 144 driven by primary winding 139 of transformer 140. The resistance of windings 142, 144 and resistors 136, 138 helps to ensure substantially equal current sharing. However, the additional resistance of resistors 120, 122, 128, 130, 136, 138 in the above-described circuits significantly reduces the efficiency of a power converter. For example, in the embodiment of FIG. 3, selecting the nominal resistance of resistors 128, 130 to be about four times the equivalent series resistance of diodes 124, 126 increases the resistive power losses by a factor of five compared to the use of diodes 124, 126 without resistors 128, 130.
There are a number of prior art circuits designed to provide equal current sharing and switching losses between power devices. U.S. Pat. No. 3,699,358 issued to Wilkinson ("Wilkinson") and U.S. Pat. No. 4,567,739 issued to Corey, et al., ("Corey, et al.") disclose the use of current transformers coupled to power switching devices. In a typical configuration, the transformer winding is coupled to either the emitter of a bipolar transistor or the source of a FET switch to force equal current sharing between the FET switches. The use of the additional transformers in Wilkinson and Corey, et al., adds significantly to the cost and size of their power converters. Also, at high switching frequencies, e.g., greater than 100 kHz, commonly used in many modem power converters, there are significant energy losses in the transformers, which further reduces the efficiency of the power converters.
Another prior art technique for providing equal current sharing and switching losses involves using "matched" power devices with similar specifications, such as FETs with similar drain to source resistance, R.sub.sd, or diodes with similar forward voltages. However, matched power devices are typically expensive, since numerous power devices must be tested and sorted in order to achieve a small subset of devices with substantially similar current-voltage characteristics. The prior art also discloses carefully balancing the printed circuit board (PCB) layouts to make the track resistance and inductance for each of the parallel power devices similar. This further equalizes current sharing and switching losses between parallel power devices. However, this technique also may lead to increased circuit cost.
While matching device characteristics and printed circuit board layouts does tend to improve current sharing, matching techniques do not guarantee current sharing is achieved so as to eliminate the possibility of thermal runaway of one of the parallel matched devices, especially at high current levels. At high current levels, each power device dissipates a substantial amount of power. Consequently, there is an increased possibility of substantial temperature increases in a power device operating at such current levels. As mentioned above, the inherent instability in high current, high power semiconductor devices results from the forward voltage negative temperature coefficient of certain power devices, such as silicon and Schottky diodes and bipolar transistors. Thus, if two or more power devices are coupled in parallel, then more current will flow through the power device having the lower forward voltage than through the power device(s) having the higher forward voltage. Also, the temperature rise of an individual power device will depend both upon its power dissipation and upon how it is thermally coupled to other elements. The device with the lowest forward voltage will tend to draw more current and operate at a slightly elevated temperature compared to other diodes. However, this will further lower its forward voltage and series resistance, leading it to draw more current as it heats. Since the power device has a forward voltage negative temperature coefficient, the temperature of the power device is fed back in a positive feedback loop that causes the current through the power device to increase further, ultimately causing thermal runaway of the power device.
Another prior art technique for providing equal switching losses between parallel power devices involves adding individual snubbers, such as those containing an inductor and a capacitor, to each of the power devices. One such technique is described in an article by A. Piekiewicz and D. Tollik entitled "Snubber Circuit and MOSFET Paralleling Considerations For High Power Boost-Based Power-Factor Correctors", which is incorporated herein by reference. This techniques helps to equalize switching losses. It does not, however, ensure equal current sharing between the parallel power devices.
The prior art also teaches mounting the parallel power devices on a single heat sink to ensure that they operate at similar temperatures so as to prevent thermal runaway of the power devices. One such technique is described in an article by Romeo Letor entitled "Static and Dynamic Behaviour of Parallel IGBTs", which is incorporated herein by reference. This technique helps equalize the temperature at which the parallel devices operate. However, it does not ensure equal current sharing and switching losses between the power devices, as it does not ensure matching of the temperature of the parallel devices.
Generally, while there are many attempted approaches to improve current sharing between parallel switches or diodes, they suffer from numerous drawbacks. Many previous approaches substantially increase the cost, size or complexity of the total circuit.
Thus, it is desirable to provide a circuit that provides equal current sharing and dissipation losses among power devices connected in parallel without suffering from the various disadvantages associated with the above described prior art techniques designed to achieve the above-mentioned equalization.